Barrel is a 10mm long 1mm calibre length of brass tube, naturally power is laughable at best, a steel BB will ricochet off a sheet of paper - but it works

edit: did some more testing cranked up to 500 psi, a 1mm BB will happily punch through 2 sheets of paper. I tried using a length of 1mm rod as ammunition to make it more visible in high speed, the heavier weight and reduced velocity made it ricochet off the paper.
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Here's a test video at 1000 frames per second:
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edit: cleaned it up on my drill-in-vice "lathe" and updated the photos.